Mine is jailbroken so I have all of the Genesis, NES, and Gameboy roms but honestly, I played Tiger Woods PGA Tour the most. It's alot of fun on the iphone because it's wonky controls without a dpad don't matter, touchscreen is the better option. I ended up going through the entire PGA tour, winning every event, and generally get at least 15 under par each round when I'm not messing around.
Not supporting Woods financially is also a plus to consumers these days.
Is the Hipstamatic camera an app or does that come standard? Either way the photos it creates are pretty sweet. My buddy has been uploading photos he's taken with it to Facebook and some of them look really cool.
If any of you baseball fans/iPhone owners happen to be literate, an app called Classics is now available free. It has, well, 'the classics' around 20 or so. Everything from Sun Tzu to Homer (no, not the one who says D'oh!) to Stoker to Twain, from Dickens to Doyle.